Re: Java 8 and type erasure

Posted by Kristoffer Sjögren on Aug 18, 2015; 12:48pm
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/Java-8-and-type-erasure-tp2411p2432.html

Do you have a link to these patches?

Reading through the thread, I get the feeling they didn't reject the
idea completely.

Considering there are also other projects (Crunch, Spark, Storm, etc)
that would benefit from this, maybe we can convince them together?


On Tue, Aug 18, 2015 at 2:27 PM, Stephan Ewen <[hidden email]> wrote:

> Yep, Timo wrote the patch, both for OpenJDK and JDT - the JDT one was
> accepted in the end.
>
>
> On Tue, Aug 18, 2015 at 2:09 PM, Robert Metzger <[hidden email]> wrote:
>>
>> Exactly, Timo opened the thread.
>>
>> On Tue, Aug 18, 2015 at 2:04 PM, Kristoffer Sjögren <[hidden email]>
>> wrote:
>>>
>>> Yeah, I think I found the thread already... by Timo Walther?
>>>
>>> On Tue, Aug 18, 2015 at 2:01 PM, Stephan Ewen <[hidden email]> wrote:
>>> > Would have been great. I had high hopes when I saw the trick with the
>>> > "constant pool", but this is only to make what Flink does already
>>> > applicable
>>> > to non-serializable lambdas.
>>> >
>>> > If you want to help us with this, I'll ping you for some support on the
>>> > OpenJDK mailing list ;-)
>>> >
>>> > On Tue, Aug 18, 2015 at 1:59 PM, Kristoffer Sjögren <[hidden email]>
>>> > wrote:
>>> >>
>>> >> I suspected that you already had looked into this, but it was worth a
>>> >> try. It would make everything so much easier.
>>> >>
>>> >> Thanks for the explanation :-)
>>> >>
>>> >>
>>> >> On Tue, Aug 18, 2015 at 1:50 PM, Stephan Ewen <[hidden email]>
>>> >> wrote:
>>> >> > Hi Kristoffer!
>>> >> >
>>> >> > I looked through the code as well. In fact, Flink currently uses the
>>> >> > trick
>>> >> > mentioned for Serializable Lambdas in the gist you sent me.
>>> >> >
>>> >> > This works well for lambdas that return simple types (primitives or
>>> >> > classes
>>> >> > without generics). The information for the generic parametrization
>>> >> > is
>>> >> > unfortunately really erased, it is in no signature or anything.
>>> >> >
>>> >> > Java has the concept of "generic method signatures", which means
>>> >> > that a
>>> >> > method gets a signature string that includes the generic types.
>>> >> > These
>>> >> > signatures are generated for regular functions, but OpenJDK and
>>> >> > OracleJDK do
>>> >> > not generate them for synthetic methods (like lambdas).
>>> >> >
>>> >> > We tried to submit a patch to OpenJDK to add these generic
>>> >> > signatures to
>>> >> > lambda methods, but they did not like the fact that we try to figure
>>> >> > out
>>> >> > the
>>> >> > generic types of lambdas. I hope they change their minds at some
>>> >> > point...
>>> >> >
>>> >> > Stephan
>>> >> >
>>> >> >
>>> >> >
>>> >> >
>>> >> > On Tue, Aug 18, 2015 at 11:46 AM, Aljoscha Krettek
>>> >> > <[hidden email]>
>>> >> > wrote:
>>> >> >>
>>> >> >> Unfortunately, this also doesn't work for the same reasons. The
>>> >> >> generic
>>> >> >> types of generic parameters of a lambda are not stored anywhere.
>>> >> >> Stephan
>>> >> >> mentioned to me that the only possibility right now would be to
>>> >> >> look at
>>> >> >> the
>>> >> >> code using something like ASM to find a cast in the code to the
>>> >> >> concrete
>>> >> >> type of the generic parameter.
>>> >> >>
>>> >> >> On Tue, 18 Aug 2015 at 11:35 Kristoffer Sjögren <[hidden email]>
>>> >> >> wrote:
>>> >> >>>
>>> >> >>> How about https://github.com/jhalterman/typetools?
>>> >> >>>
>>> >> >>> On Tue, Aug 18, 2015 at 11:16 AM, Aljoscha Krettek
>>> >> >>> <[hidden email]>
>>> >> >>> wrote:
>>> >> >>> > Hi Kristoffer,
>>> >> >>> > I'm afraid not, but maybe Timo has some further information. In
>>> >> >>> > this
>>> >> >>> > extended example we can see the problem:
>>> >> >>> > https://gist.github.com/aljoscha/84cc363d13cf1dfe9364. The
>>> >> >>> > output
>>> >> >>> > is:
>>> >> >>> >
>>> >> >>> > Type is: class
>>> >> >>> > org.apache.flink.examples.java8.wordcount.TypeTest$Thing
>>> >> >>> > class org.apache.flink.examples.java8.wordcount.TypeTest$Thing
>>> >> >>> > Type is: class
>>> >> >>> > org.apache.flink.examples.java8.wordcount.TypeTest$Thing
>>> >> >>> > class org.apache.flink.examples.java8.wordcount.TypeTest$Thing
>>> >> >>> > Type is :
>>> >> >>> >
>>> >> >>> >
>>> >> >>> >
>>> >> >>> > org.apache.flink.examples.java8.wordcount.TypeTest.org.apache.flink.examples.java8.wordcount.TypeTest$Thing<java.lang.String>
>>> >> >>> > class org.apache.flink.examples.java8.wordcount.TypeTest$Thing
>>> >> >>> >
>>> >> >>> > So with the two lambda calls it does not correctly determine the
>>> >> >>> > generic
>>> >> >>> > parameter of Thing while it works with the anonymous class.
>>> >> >>> >
>>> >> >>> > I was exited when I saw the example because we've been trying
>>> >> >>> > for so
>>> >> >>> > long to
>>> >> >>> > get this to work. Maybe we have to wait for java 19 to get this
>>> >> >>> > to
>>> >> >>> > work. ;-)
>>> >> >>> >
>>> >> >>> > Thanks a lot for helping, though. :D
>>> >> >>> > Aljoscha
>>> >> >>> >
>>> >> >>> > On Tue, 18 Aug 2015 at 11:01 Kristoffer Sjögren
>>> >> >>> > <[hidden email]>
>>> >> >>> > wrote:
>>> >> >>> >>
>>> >> >>> >> Hi
>>> >> >>> >>
>>> >> >>> >> Potential fix for writing flink jobs using lamdas without
>>> >> >>> >> Eclipse
>>> >> >>> >> JDT?
>>> >> >>> >>
>>> >> >>> >> https://gist.github.com/aslakhellesoy/3678beba60c109eacbe5
>>> >> >>> >>
>>> >> >>> >> Cheers,
>>> >> >>> >> -Kristoffer
>>> >> >
>>> >> >
>>> >
>>> >
>>
>>
>